Geolocation techniques attempt to provide an accurate determination of the current geographic location of a particular object. A common form is radiolocation, which utilizes the transmission and reception of radio frequency (RF) signals and measurements of the RF signal characteristics. In particular, the location of a transmitting source can be determined using a plurality of receivers that obtain measurements relating to the intensity (signal strength) or the phase (delay) of the transmitted RF signal. One approach uses the received signal strength (RSS) to estimate the distance to the transmitting source, assuming that the transmitted signal strength is known. Alternatively, the transmission distance can be estimated based on the time of arrival (TOA) at the receivers if the transmission time and speed of propagation are known. The angle of arrival (AOA) of the transmitted signal, which can be obtained using a directional antenna, may be combined with the distance estimate to provide the transmitter position. A further approach is known as time difference of arrival (TDOA), which is a form of multilateration that measures the differences between the arrival times at multiple spatially separated receivers situated at known locations. For each pair of receivers where their respective locations and the TDOA between them are provided, the set of possible locations for the transmitter that satisfy the measurements will form a hyperbolic curve. By utilizing additional TDOA measurements for other pairs of receivers, multiple hyperbolas can be constructed, and their intersection provides an indication for the likely location of the transmitter.
The aforementioned calculations are straightforward when assuming free-space propagation, but in real-world conditions the transmitted signal tends to undergo phenomena such as absorption, attenuation, refraction, and reflection, which influence the actual propagation of the signal. In environments with an abundance of obstacles and barriers, as is typical in urban terrain, there is a substantial degree of interference from the intervening surfaces, resulting in multipath distortion where reflected and delayed replicates of the signal arrive at the receiver antenna. The obstructed signal undergoes constructive and destructive interference along multiple paths before reaching the receiver, causing variations in both the amplitude and phase of the signal that affect the accuracy of the location estimate. If the receivers are situated at a substantial height it may be possible to bypass many potential obstructions in the transmission region and substantially avoid multipath propagation. However, certain applications are constrained to using ground receivers and thus suffer significant multipath effects.
For a TDOA radiolocation scheme that assumes free-space propagation, the received signals at a pair of receivers differ by a delay and attenuation according to the following equations:x1=α1s(t−τ1)x2−α2s(t−τ2)=α12s(t−τ12);where: x1, x2 are the signals received at the two receivers, s(t) is the signal transmitted from the target, α1, α2 are the attenuations and τ1, τ2 are the delays. α12 and τ12 represent the relative attenuation and the delay between the two sensors, respectively.
However, if the free-space propagation assumption is not valid, such as with ground-level receiver configurations, the multipath model in which each receiver receives a combination of shifted and attenuated signal reflections can be expressed by the following equations:
            x      1        =                  ∝        1            ⁢                        s          ⁡                      (                          t              -                              τ                1                                      )                          +                              ∑                          i              =              1                                      L              1                                ⁢                                          ⁢                                    α              1              i                        ⁢                          s              ⁡                              (                                  t                  -                                      τ                    1                    i                                                  )                                                          ,          ⁢                              x          2                -            ⁢              ∝        2            ⁢                        s          ⁡                      (                          t              -                              τ                2                                      )                          -                              ∑                          i              =              1                                      L              2                                ⁢                                          ⁢                                    α              2              i                        ⁢                          s              ⁡                              (                                  t                  -                                      τ                    2                    i                                                  )                                                          ;  
where: αji, τji are the attenuation and delay of the ith reflection at the jth receiver, and Lj is the total number of reflections for that receiver. The nature of the multipath which is characterized by the number of reflections, the reflections power and the reflections delays is dependent on the nature of the environment. Dense urban environments are characterized by a large number of reflections which may be stronger than the direct path signals. Even if the multipath reflection signal is relatively weak (e.g., less than 40 dB), it can still have a significant influence on the accuracy of the final location estimate.
Various approaches for mitigating the effects of multipath propagation on geolocation estimation techniques in general, and for TDOA-based geolocation techniques in particular, are known in the art. Some approaches involve different types of algorithms and computational manipulations that attempt to reduce the introduced biases.
U.S. Pat. No. 5,999,129 to Rose, entitled “Multiplatform ambiguous phase circle and TDOA protection emitter location”, is directed to the geolocation of a stationary RF signal emitter from two or more moving observer aircraft. The observers receive signals from the emitter, and the ambiguous phase difference between the signals is measured at corresponding update intervals. The observers perform pulse TOA measurements of the received signals over a predetermined clock interval, from which the TDOA of corresponding same-pulse emitter signals are calculated. A series of circular lines of position (LOP) are estimated for each observer based on the ambiguous phase differences measured and associated integer values. Hyperbolic LOPs are also computed based on the TDOA calculations. The emitter location is determined based on the intersection of the hyperbolic LOPs and the circular LOPs.
U.S. Pat. No. 7,132,981 to Roberts, entitled “Method of locating object using phase differences among multiple frequency beacons transmitted from spaced apart transmitter sites”, is directed to a technique for geolocating a mobile object in an environment where other locating systems (e.g. GPS systems) may not be expected to operate successfully. At least three transmitter sites whose geolocations are fixed and known transmit dual frequency beacons that are readily received by a mobile receiver within the environment of interest. A receiver located with the object processes the three sets of received signals by measuring phase differences among respective pairs of beacons. The object receiver processes the phase differences to obtain TDOA or TOA information, from which the location the object can be derived. Residual phase errors and frequency offsets may be calibrated out at the receiver.
U.S. Pat. No. 7,139,583 to Yamasaki et al, entitled “Positioning system and method based on time difference of arrival”, is directed to a positioning system that measures the time difference of arrival between a terminal station and a plurality of access points, by accumulating a plurality of reception timings for measurement signals which have passed through different propagation environments, and performing a statistic process with respect to the reception timing samples. In particular, each of the access points repeatedly measures a signal transmitted from the station, while varying parameters such as the time, position, or frequency. For example the measurements are performed while slightly changing the position of an antenna at an access point or the position of a station, or while the changing the frequency channel in use. Delay profiles are created from the measurement signals, and a given timing is determined on each delay profile as a reception timing sample for the signal. The reception timing samples are combined, and one reception timing is determined for each access point as a reference timing. The TDOA between the station and access points is calculated based on the reception timings, from which the station position coordinates are calculated.
U.S. Pat. No. 7,911,385 to Heuser, entitled “RF transmitter geolocation system and related methods”, is directed to a system for geolocating an RF transmitter in the presence of multipath interference. The system includes a plurality of RF receivers arranged in spaced relation, and a controller configured to generate a plurality of measurements associated with the RF transmitter. The measurements may be at least one of: frequency difference of arrival measurements, time of arrival measurements, time difference of arrival measurements, frequency of arrival measurements, and angle of arrival measurements. The controller computes a plurality of ambiguity functions based upon the measurements and due to the multipath interference, and projects the ambiguity functions onto a common geo-referenced grid. The controller detects a peak on the common geo-referenced grid indicative of a geolocation of the RF transmitter. The common geo-referenced grid may be generated based upon the position, orientation, and/or the relative movement of the RF receivers.
U.S. Pat. No. 8,049,811 to Gines et al, entitled “Method and system for locating signal emitters using iterative residual weighting”, discloses determining the location of a signal emitting device using at least three sensors separated and spaced apart from each other. Estimated location data for the device is determined for each sensor or sensor pair, based on which an estimated location is determined. The estimated location data may be based on the angle of arrival, time of arrival, time difference of arrival or the relative signal powers of the received signals. Residual values for the estimated location data is determined, based on a difference between the estimated device location and the estimated location data for a corresponding sensor or sensor pair. The residual values are converted into corresponding weights for the estimated location data for each sensor or unique sensor pair, which are weighted accordingly. The estimated device location is then updated using the weighted estimated location data.
U.S. Pat. No. 8,077,089 to Parker, entitled “Precision geolocation of moving or fixed transmitters using multiple observers”, discloses the geolocation of a moving transmitter using a plurality of fixed receiving devices and a moving receiver device. The fixed receivers obtain wavelength-scaled phase-difference measurements, providing a shape of the transmitter trajectory. The phase difference between the moving receiver and at least one fixed receiver is measured to obtain a phase error residual. An estimated starting point of the transmitter is moved to obtain a best-fit residual.